By the end of this month, the writing for the GOP nomination is likely to be on the wall.

In yesterday's Wisconsin presidential primary, Newt Gingrich got just 6% of the vote, or roughly half of Ron Paul's total. Newt is now solidly in fourth place in a four-man race, which means he won't be the VP, won't get a significant speaking slot in Tampa and won't play a kingmaker role in a brokered convention -- which won't happen anyway. Instead, Newt will soon be returning to his real talents: brilliant extemporaneous public speaking, and cashing large checks.

Yesterday will also likely be the day the music died for Rick Santorum. Wisconsin now joins Ohio, Michigan and Illinois on the list of must-win Midwestern states that Santorum actually failed to win. Yes, he did get 38% of the vote to Mitt's 43%, but that number is inflated (perhaps hugely inflated) by the fact that Wisconsin has an open primary, and many Democrats undoubtedly "crossed over" to vote for the candidate they'd most like to see face Obama.

Mitt Romney must have slept very well last night, since it capped the best week of his campaign, by far. As satisfying as it was to sweep the Wisconsin, Maryland and D.C. primaries and beat Santorum in the delegate count 90-6, the best news for Romney actually came out of the U.S. Supreme Court. Late last week the president's legal team suffered a tremendous beating in the challenge to the constitutionality of Obamacare. What once seemed unthinkable to the left now appears close to inevitable: the Supreme Court will rule that at least part of the health care law is unconstitutional.

If/when this happens, it will be a game-changer for the presidential race and a godsend to Romney. No one will care whether Romneycare was Obamacare lite, only that Obama put his political capital on the line to force Democrats to pass a bill that is not only unpopular but unconstitutional to boot. The Democratic base won't let Obama walk away from his signature accomplishment, and he'll be forced to defend and try to save it, which, frankly, is the last thing he wants to do while unemployment stubbornly remains above 8% and gasoline prices approach $5 a gallon.

By the end of this month, the writing for the GOP nomination is likely to be on the wall, particularly if Santorum loses his home state primary in Pennsylvania on April 24, which looks increasingly probable. Speculation will then turn to Romney's choice for vice president, where I expect three names to top the short list: Marco Rubio of Florida, Susana Martinez of New Mexico and Wisconsin's own Paul Ryan.

Ryan is probably the dark horse, but his high profile last week and at Romney's victory celebration could be significant. A lot will obviously happen in this state between now and June 5, but if it looks like Wisconsin may be in play in November, and Romney is feeling bold and confident, picking Paul Ryan as VP would really energize the presidential race.